Until now, ozone and chlorine have been widely used in vertical water treatment. However, persistent substances that are not decomposed by ozone or chlorine may be contained in, for example, industrial wastewater and the like. In particular, removal of dioxins, dioxane, and the like is a major problem.
In some areas, a method of removing persistent substances by combining ozone (O3) with hydrogen peroxide (H2O2) or ultraviolet light, thereby causing hydroxyl radicals (OH radicals), which are higher in activity than ozone or chlorine, to be generated in water to be treated, is in practical use. However as equipment and operation costs thereof are very high, this method is not very prevalent.
In view of this, a method has been proposed in which persistent substances are removed with high efficiency by causing OH radicals and the like generated by a discharge to act directly on water to be treated. More specifically, a treatment such as the following is being implemented.                Water to be treated containing bacteria is supplied from a water supply pipe at an upper step portion of a step-like flow path in which a plurality of step portions and a plurality of flat plate portions linked thereto are alternately connected, toward the step-like flow path.        Then, plasma generated by a plasma generation device is irradiated toward the water to be treated flowing as a water film along the step-like flow path, causing water molecules in the water to be treated to dissociate and generate O radicals and OH radicals, which eliminate the bacteria in the water.        
With this water sterilization apparatus and water sterilization method, contact between the water to be treated and the plasma is facilitated such that the bacteria in the water to be treated can be sufficiently eliminated (see PTL 1, for example).
Further, a water treatment apparatus has also been proposed in which water to be treated is treated by arranging, in an inclined state, a pair of electrode plates that vertically oppose each other so that water to be treated flows downward onto a lower electrode, and forming a barrier discharge between the electrodes. With this water treatment apparatus, the water to be treated can be treated efficiently using a simple configuration (see PTL 2, for example).